Protective circuit



De 11, 1951 F. D. GREENLEAF ETAL 2,577,848

PROTECTIVE CIRCUIT Filed Feb. 16, 1950 Patented Dec. ,11, 1951 Y 577,4

UNITED STATES 2,577,848 PROTECTIVE CIRCUIT Francis/KD. Greenleaf,Y Oaklyn,y Louis-Mr Seeberger,

' Woodbury, and'- Spurgeon H.. Buder, Merchantville, N. J., assgnors toRaIdiofCorporation'of America, a corporation of Delaware Application February 1e, 195o, seriaiNo. 144,544

` 7 Y Claims. (Cl.4 315%20) 1 2 :This invention relatesto the protection of the In accordancewith one' aspect of thisinvention, target or vscreenstructureof a cathode ray tube thecontrolof the voltageapplied tothe electrode in theleventefsweep failure, v or circuit which disables the beam of the cathode Previous protection 'circuits have been designed rayuzube on sweep-failure is effected inv such a to operate in combination with cathode raytubes 5 way that little or no ripple is present.

'in'which the cathode is at or near groundfpoten- Anotherobject of't'his invention is to provide an tial and the screen or targetis'ata-relatively high improved-protection circuit for cathode ray tubes positive potential. 'When howeverythese circuits having a cathode'that is ata negative potential areemployed in combinationl with-a cathode ray with respect'to ground in which a minimum portubein which the screen or'target'isat-ornear 10 tion of the yprotection Acircuit is'below-groun'd ground potential and the cathode is at a relatively potential.

highnegative'potential, Vcertain diiculties arise. Another object ofthe invention is to provide 'Forexampla the protection circuit-itself is ata an improved high speed'protection circuit for highly negative potential so as to necessitate cathode ray tubes having a cathode at anegative greater insulationand introduce a'safety hazard. l5 Ypotential with respect to Yground Vin which 'no Furthermore, previously suggestedcircuits perripple of sweep frequency is intro'dcedv into the .rnitri'pple that is'generally presentin' the delecintensity modulation of thev cathode ray beam. tionvoltages applied to their input terminals to These andv otherpojectswill become apparent -modulate' the-intensity'of the beam of electrons. upon detailed consideration of the drawing in In' some cathode ray tubes such asgraphecons, 20 which there is shown a schematic arrangementof even' a slightamount of extraneous beammodulaa protective circuit: 'embodying the principlesiof tion is disastrous -because or" their great sensithis invention. y K tivitv Of course, Vvlters 'having long time con- Before proceeding-'with the details of the sche- ,stants canV be employed'to reduce thea-mountoi matic circuit showntin'the drawing, the following ripple,`but this necessarily'slows downthe speed 25 'briefexplanation of the overall operation is'given. WthlWhCh the DFOSC'OD Circuit Cani-acttosuch 'A voltage waveform having theY shape ofthe cur- -an extent as to make it useless Vin Vcertainapplirent in the deflection coilsof the cathode raytube cations. vis derivedfrom thedeflection'circuit and is differ- The current owing inthe de'ection coil lcircuit entiated' so as'to generate a'large negative pulse developsa voltage which is differentiated, double 30 of'short duration lwith respect to the sweep cycle. clipped 'and' stored. lThis stored voltage is used This negativepulse is doubly clipped, the slice of to maintain'a flip-flop circuit in one state. `When the pulse between theiclippinglevels beingjemthe voltage fails the flip-flop circuit changes'to ployed" to charge' an energy storage circuit. :The the other state ofequilibrium and biases oil the output of the energy storage circuit is employed high voltage power supply. 35 to'Y maintain la :dip-:nop multivibrator Whichmay Ingeneral, there have been two differenttypes be of Athellccles 'Jordantype on one of' its states of circuits employed for protection of thev cathode of operation, r 'lhejnip-'flop circuit is connected ray tubes inthe eventofsweep failure. In `afirst in'such manner" that whenthe deflection circuits type, avoltageis derived from the sweep circuits fail theip-ilop circuit reverts 'to its other state.

Yandapplied to'an electrode v`or circuit associated 40 The output of the nip-'flop circuit isapplied to with the operation of the tube. In asecond type, control an oscillator in such `aA-Way f that when a'voltage'is derived independently 4ofthe lsweep itisin its lotherstate," the output of the oscillator circuitsandlis applied to an electrode `orV circuits is; reduced to Zero. jkSince the outputof the oscilassociatedwith the operation of the'tube'under lator is're'ctiedi'sofas to provide an 'accelerating the control ofjvoltages derived from the sweep @potential to'one ofthev electrodes of th'e'cathode circuits. 'Inbothtypes the-electrode or circuit ray tube, the 'drop' A'in its "output voltageservesto associated with the Cathode lay tube todhich reduge theacqelerating'potential applied t0 this these'varously 'derivedvoltaees arev appledoperelectrode and -therefore'intensity of the'beamin ates upon the failure of such voltage -to disable theeathoderay tubeisiredueed,

the beam, thus preventing damage to the ltube. -50 The drawing shows a'circuit arrangement that This may be done by defocusing `or=reducingthe is' adapted to protectthe screen-or targetvoffa number of electrons in the beam. fcatho'de ray tube sini/the eventfof failure in the The first/'type of protective circuit-may be usef 'sweep circuits in accordance with the principles vin applications where either agreat deal of ripple offthisinvention. Resistor 2 is inserted inlseries and fastaction or a-sm'all'amount Vof rippleand withthe'deection coils soV thatI a voltage having slow #action can be tolerated. As pointedout al sawtooth vWaveform such "las indicated byi'nuabove, this invention is directed towards Vapplicafineral v-iiisv developed `across it only when ithe tions, `such vas in a graphecon, Awhere both fast deilectio'n coils are operating to deflect the beam.

vaction and freedom from ripple are required. YVoltage waveform'fil is applied in'a negative- Thisinvention may be said to belong to the sec- @o 'goingsen'setofthefgridl tof the'fampli'er Bivia -ond-`of the two'typesof protectivezcircuits mencondensersl'and'lf! and grid leak resistor I4. .tionedabove "'-Waveform 4f therefore `appears `Tat the plate `lli `to the grid 38 of amplier 40.

of amplifier 8 in inverted form and is coupled to the grid i8 of amplier 20 via condenser 22 and resistor 24. The values of the condenser 22 and resistor 24 are chosen in accordance with the principles well known to those skilled in the art so that they differentiate the inverted waveform 4 that appears at the plate |6 of the ampliiier 8. In this way, the negative-going portion of the waveform is accentuated so that the waveform 26 is applied to the grid I8. It will be noted that the negative peaks 28 of this wave form are of short duration with respect to the sweep cycle. Because waveform 26 is coupled to the grid I6 via condenser 22, and because tube 26 is operated without cathode bias, waveform 26 will be clamped to Zero volts at the axis shown by dotted line 36. The amplitude of waveform 26 is sufficient so that plate current cut o for tube occurs at axis 34 of waveform 26. In this way, variations in the amplitude of the waveform 26 will not pass through ampliiier 20 and the reason for this will become apparent as the description proceeds.

After being limited and having its phase inverted in ampliiier 20, waveform 26 is applied Again clamping of the clipped waveform 26 occurs at grid 38 because of the absence of cathode bias in tube and the amplitude is such that clipping of waveform 26 occurs at axis 43. The double clipping action thus performed insures that the pulses supplied by the amplier 40 via condenser l4i over lead 44 have a uniform amplitude regardless of variations in amplitude of waveform 26.

Lead 44 is connected to the cathode 46 of unilateral conducting device 48 and is returned to ground potential via resistors 50 and 52, the resistor 56 being considerably larger than resistor 52. The resistor 52 is connected in series with the resistor 54 and this series combination is connected between ground and a source of fixed positive potential so that the cathode 46 of the unilateral conducting device 48 is maintained at positive xed potential with respect to ground in the absence of any signal on lead 44.

The plate 56 of the unilateral conducting de'- `vice 48 is connected to the junction of resistor 52 and 54 via resistor 60 and condenser 62 connected in parallel. On the absence of any signals on lead 44 therefore, plate 56 will have the same positive potential as the cathode 46 and no current ows through the unilateral device 46.

Upon the appearance of the negative pulse occurring between leads 43 and 34 of waveform 26 on lead 44, the cathode 46 becomes negative with respect to plate 56 because most of the potential drop appears across resistor 50. The unilateral conducting device 48 then charges condenser 62 so that the right hand side thereof as shown becomes negative during normal operations, that is, when the negative pulses 28 of waveform 26 appear at uniformly spaced intervals. The values of the above circuit components are such that cathode 46 is maintained at an average positive potential with respect to the plate 56 of the unilateral conducting device 4B. However, during the negative pulses appearing on lead 44 the cathode 46 becomes negativel with respect to the plate so as to permit the unilateral conducting device to pass current. At the same time, the plate 56 of the unilateral conducting device 48 is driven in a positive direction by the discharge of condenser 62 through resistor 60. If the time constant of the con denser 45 and resistor 50 is sufficiently large with respect to a sweep cycle, then the unilateral conducting device 48 will be cut off during the interval between the negative pulses 28 of waveform 26. During this interval, the voltage appearing at the plate 56 of the unilateral conducting device 48 is determined by the time constant of condenser 62 and resistor 60 alone.

The voltage waveform appearing at plate 56 is generally indicated by the numeral 64, the sharply sloped edge 66 occurring during the negative pulse 28 of waveform 26 and the more slowly sloping side 68 occurring during the discharge of condenser 62 through resistor 60. The waveform 64 is coupled to the grid 10 of tube 2 via condenser 14 and resistor 16, the values of which are so chosen that their time con.- stant is shortl in comparison with a sweep cycle. Therefore, the waveform appearing at grid 10 follows closely that appearing at plate 56. Tube 'd'2 must be comprised of a dual triode with heaters in series or with a common cathode, plate 'i8 being associated with grid 10, and plate being associated with grid 82. Cathode 84 is common to both triode sections and is connected to ground by a suitably by-passed resistor 86. rihe plate 18 is connected to a source of fixed potential Via plate load resistor 88 and plate 80 in the right hand half of the tube 12 is connected to the same xed potential via a load resistor 06 which is small in comparison with load resister 68. The plate '|'8 of the left hand half of tube i2 is conected va voltage dropping resistors 62 and 94 to the grid 82 of the right hand half of the tube '|2 so as to bias it more positively than the grid 'i6 of the left hand half of the tube l2. In this way, the right hand half of the tube 'l2 remains in a conducting condition and the current drawn through the cathode resistor 86 is sufficiently positive to aid in cutting off the left hand half of the tube 12.

As long as the sweeping operation is performed in normal fashion, the negative potential created by the discharge of condenser 62 maintains grid i0 at sumciently negative potential to keep the left hand half of tube '|2 cut oi. The time constant of the RC combination of condenser 62 and 60 is suitably chosen so that this condition persists for at least, but not much more than, the remainder of the cycle following the negative pulses 28 of waveform 26. As an illustration, line 96 in waveform 64 indicates the potential above which grid *i0 will reverse the conducting conditions of the tube 12 so that the left hand half of the tube becomes conductive Q and the right hand half of the tube is cut off.

The common cathode 84 of the tube 12 is coupled by lead 68 via current limiting resistor |60, tank circuit |02 and resistor |04 to the grid |06 of an oscillator generally indicated by the numeral |06. This oscillator may be of any standard type, and itis self-biased by condenser v ||6 in parallel with the resistor |04. A voltage regulator tube ||2 and a diode ||4 are connected in series between B- and lead 98 so as f to be in parallel with the cathode resistor 86 of tube 12. The cathode ||6 of the oscillator is connected to the junction between voltage regulator tube ||2 and diode ||4. In this way, the bias between the grid |06 and the cathode llt` of the oscillator |68 is not affected by the voltage appearing across resistor 86 of flip-flop 12 as long as that voltage is greater than that appearing across tube i I2. Any positive pulses appearing across cathode resistor 86 will be in effect short-circuited to the potential established by the voltage regulator 2 by the presence of the diode H4. Though voltage regulator tube ||2 is not essential to the operation of this invention, it helps to stabilize the oscillator |08.

The output of the oscillator |08 is coupled to a rectifier by the inductive coupling between coil |22 and coil |24 that is in the tank circuit of the oscillator. The plate |26 of the rectifier |29 is connected to one end of coil |22 and the cathode |28 of rectifier |20 is connected to the other end of coil |22 Via a suitable load circuit generally indicated by the numeral |30. The load circuit |30 may be made in any way known to those skilled in the art, but in this illustrative example, it is comprised of a condenser |32 connected between the cathode |28 of rectiiier |20 and the other end of coil |22, a resistor |34 and condenser |36 connected in series parallel relationship with condenser |32, and a resistor |38 connected in parallel with condenser |36.

A series oi neon tubes |40, |42, |44 and |46 are connected in parallel with resistor |38 and condenser |36. The lower terminal of neon tube |46 or the other side of coil |22 is connected to the highly negative cathode of thecathode ray tube. In the arrangement shown, the heater for the cathode of the cathode ray tube is connected in parallel with the heater for rectifier |20 and the cathode of the cathode ray tube is connected to its heater. junction of resistor |30, condenser |36, resistor |38 and neon tube |40 will be maintained at a constant fixed potential above the highly negative potential of the cathode ray tube. Therefore, when this junction is connected to an accelerating electrode of a cathode ray tube, an electron beam is permitted to exert.

The time constant of the load circuit |30 of rectier |29 need only be long in comparison with a half cycle of the frequency of oscillator |03 in order to prevent any ripple from appearing on the accelerating electrode on the cathode ray tube. As the frequency of the oscillator |08 can be made extremely high, this means that the time constant of the load circuit |30 of rectier |20 can be made extremely short in comparison with the sweep cycle of the beam deiiection system associated with the cathode ray tube. The advantage derived from this is that when failure of the deflection circuits is indicated by a drastic drop in the output of oscillator |08, the voltage applied to the accelerating electrode of the cathode ray tube can drop with great speed and therefore cut oi the beam before any damage can be done to the screen or the target of the cathode ray tube.

Having described our invention, claim is:

1. A protection circuit for cathode ray tubes comprising in combination a deection circuit adapted to operate at a cyclic rate, means for developing a direct voltage pulse once during each cycle of operation of said deiiection circuit, a trigger circuit, a coupling network for connecting said means for developing pulses to said trgger circuit in such manner that the pulses tend to maintain said trigger circuit in one state, the time constant of said coupling network being such that said trigger circuit is maintained in one state for at least the remainder of one cycle of operation of said deection circuit, a freerunning oscillator, a coupling circuit connected between said oscillator and said trigger circuit in such manner that oscillations are interrupted when the trigger circuit is in its other state,

what we In this way, the potential at the and a rectifier for deriving a direct current potential in response to the output of said oscillator.

2. A protection circuit such as described in claim l in which said trigger circuit is comprised of at least a first and a second electron discharge device, the rst being cut off and the second conducting when the trigger circuit is in one of its states, said coupling network being connected so that said pulses are applied in a negative-going sense to said rst electron discharge device, thus tending to prevent any deflection voltage from modulating said oscillator during normal operation.

3. A circuit for protecting cathode ray tubes in the event of sweep failure comprising in combination a sweep circuit adapted to operate at a predetermined cyclic rate, means for developing a pulse having a short duration in comparison to a cycle of said sweep circuit, a trigger circuit having an output terminal that is at a given fixed potential when said trigger circuit is in one state and at a relatively negative fixed potential when said trigger circuit is in the other state, a coupling network for applying said pulses to said trigger circuit in such manner as to maintain it in the other state, the time constant of said coupling circuit being such that the trigger circuit is maintained in one state for at least the remainder of one cycle following each pulse, a free running oscillator comprising an electron discharge device, a control electrode associated with said discharge device, said control electrode being connected to said terminal, and a rectifier adapted to produce a xed potential in response to the oscillations provided by said oscillator.

4. A device such as described in claim 1 in which said coupling circuit comprises a diode and a voltage regulator tube connected in series.

5. A protective device such as described in claim 2 in which said rst and second electron discharge devices have a common cathode.

6. A protective device in accordance with claim 2 and wherein said iirst and second electrode discharge devices each have a filament and wherein said filaments are electrically connected in series.

7. A protective device such as described in claim 1 in which said coupling circuit; includes a clamping circuit of such polarity that positive pulses are prevented from reaching the oscillator.

FRANCIS D. GREENLEAF. LOUIS M. SEEBERGER. SPURGEON H. BUDER.

REFERENCES CITED The following references are of record in the file of this patent:

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